The invention is related to the field of servo compensation for disk storage systems, and in particular, to a feedback control system that corrects spin-frequency harmonic run-out error and operates as an oscillator.
Disk storage systems store data in circular tracks on a disk that is typically a is magnetic disk or an optical disk. To read or write data to a magnetic disk, a servo positioning system positions a head over a track as the disk spins. Ideally, the servo positioning system would accurately position the head over the desired track, but disk storage systems experience servo position errors that cause a misalignment between the head and the track. Positioning occurs in two modesxe2x80x94seek and follow. In the seek mode, the head is moved from one track to a desired track. In the follow mode, the head remains positioned over the desired track.
Disk storage systems correct servo position errors using a servo compensation system. The servo compensation system uses a feedback control loop to monitor the past performance of a system and make on-going adjustments to correct errors. The feedback control loop has a position error detection system that generates a position error signal using information obtained in current and previous read operations of servo data. A typical position error detection system is comprised of the following components coupled in series: a head, an amplifier, a read channel, a servo demodulator, an analog to digital converter, and subtraction circuit. The feedback control loop has a servo compensation system that processes the position error to generate a servo compensation signal. A typical servo compensation system is a digital filter. The feedback control loop also has a servo positioning system that adjusts the position of the head based on the servo compensation signal. A typical servo positioning system is comprised of the following components coupled in series: a digital to analog converter, a voice coil motor amplifier, a voice coil motor, and a voice coil motor actuator.
A specific type of error, referred to herein as xe2x80x9cspin-frequency harmonic run-out errorxe2x80x9d, occurs when the disk or the circular tracks on the disk do not spin perfectly about the central axis of the disk. Spin-frequency harmonic run-out error has sinusoidal characteristics with associated harmonics because it tracks the spin of the disk. A large proportion of the spin-frequency harmonic run-out error is generated at the first harmonic of the spin frequency. However, spin-frequency harmonic run-out error is also generated at the second harmonic of twice the spin frequency and at additional harmonics. Spin-frequency harmonic run-out error generated at the second harmonic is especially problematic with removable disks, such as floppy disks, because of additional deformation in the floppy-type disk.
Some existing servo compensation systems are designed with a finite gain at the spin-frequency harmonic run-out error frequency and do not adequately eliminate spin-frequency harmonic run-out. Typical open-loop gain at the spin-frequency harmonic run-out error frequency is around 20 dB which is not enough to compensate for the spin-frequency harmonic run-out errors experienced by typical disk memory systems. Typical open-loop gain at the second spin-frequency harmonic run-out error frequency is around 10 dB which may not be enough to compensate for the second spin-frequency harmonic run-out errors experienced by typical disk memory systems. In addition, these systems are not specifically designed to compensate for spin-frequency harmonic run-out error at the second harmonic.
Existing spin-frequency harmonic run-out compensation systems are coupled to the primary servo compensation systems and are not integrated within the primary servo compensation systems. The two separate compensation systems require more hardware and can interfere with one another.
Some existing spin-frequency harmonic run-out compensation systems must be initialized when the system changes from the seek mode to the follow mode. System initialization is undesirable because it takes time for the resulting transient responses to die out and has significant consequences on system performance.
One existing spin-frequency harmonic run-out compensation system adds a feed-forward signal to the servo compensation signal. The feed-forward signal is not based on feedback, but is estimated based on previously measured spin-frequency harmonic run-out error. Estimating expected spin-frequency harmonic run-out error cancellation requires complex mathematical modeling that often allows some spin-frequency harmonic run-out error to remain. When the spin-frequency harmonic run-out error changes, overall system operation must be interrupted so the feed-forward signal can be re-calibrated. Additional hardware, firmware, computation time, and memory are required to generate the feed-forward signal.
A second existing spin-frequency harmonic run-out compensation system uses feedback to generate a separate spin-frequency harmonic run-out compensation signal that is added to the primary servo compensation signal. This spin-frequency harmonic run-out compensation system reduces spin-frequency harmonic run-out error, however, the second feedback loop formed by the spin-frequency harmonic run-out compensation system interferes with the performance of the primary the servo compensation system. In addition, the separate spin-frequency harmonic run-out compensation system requires additional hardware, firmware, computation time, and memory.
A third existing spin-frequency harmonic run-out compensation system processes the servo compensation signal with a bandpass filter. This spin-frequency harmonic run-out compensation system reduces spin-frequency harmonic run-out error, however, it does not eliminate spin-frequency harmonic run-out error because it does not have infinite gain at the spin frequency.
Existing spin-frequency harmonic run-out compensation systems must be added to the primary servo compensation system and are not integrated within the primary servo compensation system. Existing spin-frequency harmonic run-out compensation systems fail to specifically compensate for spin-frequency harmonic run-out error at the second harmonic. Some of these existing systems do not provide compensation during the seek mode and must be initialized at the beginning of each follow mode. For these reasons, there is a need for an integrated servo compensation system that eliminates spin-frequency harmonic run-out error at the first and second harmonics, and that operates during both the seek and follow modes to avoid initialization.
The invention overcomes the problems discussed above by providing an integrated servo compensation system for a disk storage system. The servo compensation system eliminates spin-frequency harmonic run-out error at the first harmonic during the follow mode, and in some examples of the invention, at multiples of the first harmonic. During the seek mode, the invention provides an oscillator at the spin frequency, and in some examples of the invention, at multiples of the spin frequency. The invention does not require initialization because it operates during both the seek and the follow modes.
The servo compensation system is comprised of a compensation means that processes a position error signal during follow mode. The position error signal is comprised of components representative of the spin-frequency harmonic run-out error and components representative of other servo position errors. The compensation means generates a compensation signal responsive to the position error signal. The compensation signal is comprised of components that cause the servo compensation system to compensate for the first and second harmonics of the spin-frequency harmonic run-out error and to compensate the other servo position errors. The servo compensation system also includes an oscillating means. The oscillating means generates an oscillating signal at the spin frequency of the disk, and in some examples of the invention, at multiples of the spin frequency. Since the spin frequency is equivalent to the spin-frequency harmonic run-out error frequency, the oscillating signal can be maintained to reduce spin-frequency harmonic run-out error at multiple harmonics during the seek mode.
The servo compensation system is based on a unique digital filter design. The design requirement combines solutions for first and second spin-frequency harmonic run-out error and other servo position errors into a single difference equation. The difference equation is solved to eliminate spin-frequency harmonic run-out error at the first and second harmonics by providing infinite gain at the spin frequency and twice the spin frequency. This design requirement requires application of the final value theorem, but before the final value theorem can be applied, new poles are added to offset existing poles caused by the sinusoidal spin-frequency harmonic run-out error. New zeros are then added near the new poles to stabilize the design.
The firmware implementation of the filter design uses an approach that departs from standard implementations. Compensation signals are generated for the proportional path, the differential path, the integral path, the spin frequency path, and the double spin frequency path. These signals are then summed to provide a single compensation signal. The firmware characteristics are then solved against the design requirements to finalize the filter design.
One advantage of the design is that both spin-frequency harmonic run-out error and other servo position errors are handled by a single compensation system. This results in less complexity and in the elimination of separate compensation systems. Another advantage of the design is that the compensation system can be operated as an oscillator during seek mode because of the additional poles. This feature avoids initialization during a mode switch. In addition, the design eliminates spin-frequency harmonic run-out error at both the first and second harmonics. The elimination of second spin-frequency harmonic run-out error is especially valuable with respect to floppy disk systems. Another advantage is that the compensator design can be easily extended to include run-out compensation at any number of higher harmonic frequencies simply by including additional parallel blocks in the compensator.